Substituents isotopic shifts

The overall biosynthetic pathway to the tetracychnes has been reviewed (74). Studies (75—78) utilising labeled acetate and malonate and nmr analysis of the isolated oxytetracycline (2), have demonstrated the exclusive malonate origin of the tetracycline carbon skeleton, the carboxamide substituent, and the folding mode of the polyketide chain. Feeding experiments using [1- 02] acetate and analysis of the nmr isotope shift effects, led to the location of... 

A series of stannylallene derivatives (111) was studied by means of 13C, 29Si and 119Sn NMR spectroscopy by Lukevics and coworkers65. The effects of substituents on chemical shift values and J (SSCC) in 111 are additive. A set of linear correlations between the isotope shifts (IS) and SSCC for 111 demonstrates the interrelation of these values. 

Nearly all the data of Sect. II was gathered from NMR spectra. In this section we turn our attention to the hydrogen bonding proton of the ds enol tautomer, looking firstly at the more usual properties of < (OHO), line width, and coupling constants. The second part will cover the substituent effects on the and 13C NMR chemical shift. The third part will deal with the isotopic shift Ad H, 2H). 

Vibrational spectroscopy has not been extensively used in the characterization of tris(dithiolene) metal complexes. Moreover, complete assignments based on both IR and Raman spectra, isotope shifts, and normal mode calculations are not available for any individual complex. However, the available data suggest that the trends in M S and dithiolene ligand vibrational modes as a function of the metal, the charge on the complex, and dithiolene substituents, closely parallel those discussed above for square-plane bis(dithiolene) metal complexes. Accordingly, the vibrational data are consistent with highly delocalized complexes with predominantly ligand-based redox chemistry. 

Stichtemath, A., R. Schweitzer-Stenner, W. Dreybrodt, R.S.W. Mak, X-Y. Li, L.D. Sparks, J.A. Shelnutt, C.J. Medforth, and K.M. Smith (1993). Macrocycle and substituent vibrational modes of nonplanar nickel(II) octaethyltetraphenylporphyrin from its resonance Raman, near-infrared-excited FT Raman, and FT-IR spectra and deuterium isotope shifts. J. Phys. Chem. 97, 3701. 

Li, X., R.S. Czemuzewicz, J.R. Kincaid, P. Stein, and T.G. Spiro (1990). Consistent p orphyrin force field. 2. Nickel octaethylporphyrin skeletal and substituents mode assigrunent from N, meso-d4, and methylene-di6 Raman and infrared isotope shifts. J. Phys. Chem. 94, 47. 

The nmr spectra of C-1 -dideuteriated [35] showed two peaks for the C-1 and C-2-methylene groups. The quintet of the deuteriated methylene C-1 was shifted upheld by 0.791-0.758 ppm compared to the corresponding protio compound, whereas the nondeuteriated methylene group C-2 was shifted downheld by 0.121, 0.060 and 0.045 ppm depending on the C-3 substituent. All other signals showed intrinsie isotope shifts upheld. 

Bishop (42) has summarized the more recent evidence, from which it also appears that isotopic shifts are not confined to protons bonded to the same carbon atom but are transmitted, considerably damped, to substituents on the adjacent carbon atom, and are thus reminiscent of inductive effects. In magnitude, the different effects vary, but qualitatively all the evidence from NMR chemical shifts confirms the greater effective electropositivity of deuterium. 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

Intramolecular nucleophilic attack on the a-position of the vinylidene complex 148 affords a zwitterionic species (149). Next, a formal 1,3-H-shift must occur before release of the observed product (146). Based on the results of an isotope labeling study, this process is believed to occur via two discrete 1,2-H-migration events (149—>150—>147). Interestingly, no back-migration of heteroatom substituents is observed [50]. 

The nature and position of substituents relative to the ring oxygen atom have an important effect on the acid-catalyzed isomerization of oxepins. It has been observed that, in addition to the hydrogen isotopes ( H, 2H, 3H), chloro, bromo, methyl (72E1129) and alkoxycarbonyl (79JA2470) substituents also show the migration-retention sequence found in the NIH shift. 

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